How to add a working navigation mesh and path finding to a maze in blender game engine?
Your mesh is not recognized by blender.It pops an error.
It seems like it was corrupted.

I tried changing it by deleting all the top vertices(there are also 4 vertices I think a bit missplaced) but even with the basic scaled plane, the same error pops.

I created a new plane in your file and it worked just fine. I don't know if it is fixable without remaking it from scratch though.

This guy here disagrees: same error